Method and plant for neutralizing acid smokes issued particularly from the combustion of residues

ABSTRACT

The invention relates to a process and an installation for neutralizing acid fumes coming in particular from the combustion of residues. 
     According to the invention, the combustion vapor and fumes laden with anhydrides and acids are neutralized after passage in the exchanger (19) by counter-flow encounter with a flow of a neutralizing liquid of basic pH constituted by a slurry formed on the one hand by water and on the other hand by the ashes coming from said combustion furnace and placed in suspension and/or solution in said aqueous phase.

The present invention relates to a process and to an installation forcarrying out this process, for the purpose of optimally recoveringthermal energy from residues and waste.

The invention relates more especially to the treatment of residues andwaste of agricultural or industrial origin as well as waste of urbanorigin such as household refuse.

The treatment of household refuse by incineration with a view toproducing thermal energy, and of industrial or agricultural waste, isone of the ways allowing on the one hand energy saving and on the otherhand the evacuation of polluting waste.

However, the treatment of waste by incineration raises numerousproblems, particularly from the standpoint of ecology.

In fact, incineration ovens supplied with various waste, particularly inthe case of household refuse, lead to burning by-products in bulk whichare difficult to separate, such as products of synthetic origin; now,these latter deliver in the combustion gases an appreciable proportionof corrosive acid vapours.

These corrosive vapours cannot be rejected into the atmosphere and it istherefore necessary, in order to avoid this drawback, to provide eithergas dilution or fume washing installations.

However, although these installations allow rejection into theatmosphere of fumes freed of an excessive percentage of noxious vapours,they produce acid water which must then be treated chemically beforebeing able to be rejected into the environment.

And under these conditions, the installations proceeding by incinerationof waste are limited economically due to the problems of pollution thatthey in turn raise and which must be solved.

The invention relates to an original process presenting the advantage ofproviding neutralization of the acid vapours escaping in the combustiongases, consequently allowing rejection of these gases into theatmosphere without creating as polluting by-products the presence ofacids in liquid phase which would previously have to be chemicallyneutralized.

To that end, the invention relates in the first place to a process forthe exploitation of residues or waste of industrial, agricultural orurban origin, with a view to exploiting the calorific power of theseresidues and proceeding by incineration thereof in a combustion furnaceserving at least one heat exchanger for the production of thermal energyin the form of high-temperature heat-transfer fluid, and the process ischaracterized in that the combustion vapours and fumes laden withanhydrides and acids are neutralized after passage in the exchanger bythe counter-flow encounter with a flow of a neutralizing liquid of basicpH constituted by a slurry formed on the one hand by water and on theother hand by the ashes coming from said combustion furnace and placedin suspension and/or solution in said aqueous phase.

According to one development of the invention, the fumes leaving theneutralization phase are subjected to condensation in order to eliminatethe water vapour, the water coming from this condensation beingrecovered in order to constitute with the ashes the neutralizationslurry.

According to another characteristic of the process, the residuesconstituting the starting raw material are subjected to an operation ofgranulometric separation, for example in a sieve drum with a view toeliminating the finest particles constituted mainly by organic matter,only the coarser elements forming the oversize and constituted by inertcombustible matter being used for supplying the incineration furnace.

And according to another characteristic, the phase of condensation ofthe purified fumes is provided with recycling of the hot water comingfrom condensation of the vapours below the dew point and this water isrecycled in order to encounter, in counter-flow, the purified vapours,the hot water supplying before its recycling at least onelow-temperature heat exchanger to which it transfers its calories.

The invention also relates to an installation for carrying out theprocess, this installation comprising an incineration furnace providedwith a post-combustion chamber which serves a heat exchanger such as atubular boiler in which the combustion gases allow the production ofthermal energy conveyed by a heat-transfer fluid such as dry steam orsuperheated water, the heat exchanger being connected to aneutralization chamber at which arrive the combustion gases leaving theexchanger, said neutralization chamber comprising at its inlet at leastone pipe for spraying a basic slurry, the purified fumes leaving theneutralization chamber arriving at a condenser constituted by a packedcolumn supplied at its top with low-temperature trickling water takingthe fumes to a temperature equal to or less than the dew point, causingcondensation of the water vapour contained in the fumes, the tricklingwater reheated by heat exchange and increased by the condensation water,at a temperature slightly less than the dew point, being collected atthe base of said packing column and being recycled at the top of thecolumn after passage through at least one heat exchanger to which ittransfers its calories, the installation further comprising a vat formixing and forming the neutralization slurry supplied with condensationwater of the purified fumes from the condenser formed by the packingcolumn and the vat being supplied by a conveyer with combustion ashesfrom the incineration furnace, said vat being connected by a recyclingcircuit to the spray pipe located at the inlet and at the top of theneutralization chamber and comprising means for evacuating the solidresidues decanted at the base of the vat.

Furthermore, the solid phase constituted by the finest particles havingpassed a step of sieving upstream of incineration is recovered in orderto constitute an organo-mineral improvement or fertilizer by possibleenrichment with mineral principles thanks to the incorporation of theashes coming from combustion in the liquid phase constituted by theslurry for neutralizing the acid vapours in the neutralization chamber.

The installation preferably further comprises a mixer adapted to receivethe finest particles having passed the sieving means located upstream ofthe incineration furnace and which are conveyed towards the mixer byconveying means from said sieve, the mixer also being connected byconveying means to the base of a vat containing the neutralizationslurries and thus adapted to convey the solid extracts sedimented insaid vat up to the mixer.

According to another characteristic, the installation assembly comprisesprogrammable control means adapted automatically to control all theoperational parameters of the installation, in particular, supply of theincineration furnace with combustible matter, supply of the combustionand post-combustion burner(s), supply and recycling of the basic slurrybetween the corresponding vat and the neutralization chamber, supply andrecycling of the evaporator for condensation of the filtration liquorsof the digestate, the circulators interposed on the fluid circuits, themotorizations of the means for conveying the solid and muddy phases.

Other characteristics and advantages of the invention will appear fromthe following description which is given in connection with a particularembodiment presented by way of non-limiting example and in the light ofthe accompanying drawings.

FIG. 1 shows a view of a general diagram of the incineration operationsfollowed by neutralization of the combustion gases.

FIG. 2 shows a view in transverse section of the neutralization chamber.

FIG. 3 shows a view in detail of the baffle walls equipping theneutralization chamber.

In all three Figures, the starting material used within the scope of theinstallation and the process according to the invention is constitutedby residues of industrial or agricultural origin or of urban residuescoming from the collection of household refuse.

These residues are placed in a heap 1 after having been subjected to anoperation of removal of metals and scrap iron by known electro-magneticmeans (not shown).

The raw material is taken from the heap 1 and (after possible treatmentin a crusher) conveyed towards a sieving or separating assembly possiblyconstituted, as indicated by way of example in FIG. 1, by a first rotarysieve drum 10; that part intended to constitute the combustiblesubjected to incineration and constituting the oversize is conveyedtowards the supply hopper 14 serving a supply conveyor 15, for exampleof the Archimedean screw type, or any other apparatus thus controllingsupply of the incineration furnace 16.

Combustion in this furnace is controlled so as to respect temperaturereferences at each step of their oxidation by the presence of burnersand of devices for regulating the combustion-supporting air.

This furnace comprises a post-combustion chamber 17 and burners (notshown) supplied with fluid combustible (gas, fuel) from an independentoutside source 18.

And the supply of this complementary combustible makes it possible toobtain perfect regularity in the operation of the assembly, compensatingthe variations in the calorific powers of the successive charges of theprincipal combustible.

The combustion gases leaving the post-combustion chamber 17 at atemperature of the order of 900° to 1000° C., pass in a first heatexchanger formed for example by a tubular boiler 19 producing at 18 aheat-transfer fluid at high temperature such as dry steam or superheatedwater or any other thermal fluid; this heat-transfer liquid is directedtowards the possible uses such as a turbine for producing electricalenergy or any conventional use.

The combustion gases leaving the exchanger 19 at a temperature of theorder of 250° to 350° C. cannot be rejected into the atmosphere as theyare considerably laden with anhydrides or acid vapours (NO₂, HCl andSO₃), these noxious products generally coming from the decomposition andcombustion of plastics materials and generally from synthetic materials(in particular PVC).

It is therefore impossible to reject these fumes into the atmosphere;furthermore, it is desirable to recover the latent heat and the sensibleheat contained in these fumes which, on leaving the boiler 19, are stillat a high temperature.

The invention solves this problem with elegance by ensuring washing ofthe fumes and neutralization thereof at high temperature before thesefumes are cooled to reach the dew point, beyond which the corrosiveacids would be formed, which are aggressive for the metals constitutingthe enclosures of the installation.

To this end, the combustion gases are treated and neutralized in aneutralizing assembly formed by the neutralizing chamber 20 associatedwith a lower vat 21.

At the top of the neutralizing chamber 20 there are provided one or morepipes 22 spraying into the chamber containing the combustion gases intransit, a rain of a liquid constituting a neutralizing slurry of basicpH and coming from vat 21.

This slurry forms a bath in the lower part of the chamber 20 from whichit is recycled by the recycling circuit 23 with pump 24'.

The basic neutralizing slurry which is permanently recycled receivesextra liquid from the effluent 31 coming from the condensation tower 25described hereinafter and supplying the vat 21 and with which areincorporated the ashes coming from the incineration furnace 16 conveyedfrom the conveying circuit 26.

A possibly intermittent mixer 30 ensures homogeneity of the assemblywithin the vat 21 whilst the particles sedimented to the base of the vat21 are conveyed by conveyer 27 towards a filter press 28 or anothersystem of separation from which the filtration liquid for example isrecycled at 29. This filter press 28 also receives via circuit 32 thesludge sedimented to the base of the neutralizing chamber 20.

The combustible supplying the incineration furnace generally includes anappreciable percentage of residues (paper, packing, cardboard)comprising a high proportion of inorganic charges leading to apercentage of ashes of the order of 10 to 16%; these inorganic ashes areformed by salts containing a considerable proportion of calcium andpotassium oxides and are therefore strongly basic. These ashes arepoured into the liquid phase of the vat 21 where they are placed insuspension or (totally or partially) in solution to constitute a slurrywith strongly basic pH which may be taken, for example, from aperipheral chute or any other system by the circuit 23 arriving at thespray pipe 22.

Within the chamber, the liquid phase formed by the basic slurrydescribed hereinabove encounters the acid vapours which are neutralizedby leading to the formation of generally insoluble salts which sedimentin the bottom of the chamber 20 to arrive for example via the evacuationcircuit 32 and at the filter press 28.

FIG. 2 gives a detailed view of the constitution of the neutralizingchamber 20.

The fumes laden with acid vapours arrive through the supply opening 33and spread in the whole of the enclosure constituted by the chamber 20.

However, from the supply opening 33, the vapours pass through a firstcurtain of neutralizing slurry poured from pipe 22'.

This pipe is positioned at the opening of pipe 33 and it allows a firstprecipitation of the insoluble salts which are formed rapidly beinggiven the thermal shock to which are subjected the vapours which arriveat a temperature of between 200° and 300° and which then encounter acurtain of rain at a temperature close to ambient temperature.

The fumes then traverse the chamber 20 in an eddying path provoked bythe presence of the baffle devices 34, 35, 36.

Each of these devices is itself constituted by two walls 37, 38 (FIG.3), these walls being provided with openings 39, 40, 39', 40' which aredisposed in quincunx from one wall to the other.

It is seen that, under these conditions, the gaseous streams which havepassed through an opening 39, 40 of one wall 37 arrive in theinterstitial space between the two walls 37 and 38 without encounteringthere opposite a corresponding opening.

The gaseous streams are therefore deviated and led to follow a pathgenerating eddies (as shown in FIG. 3) before flowing through the secondset of openings 39', 40' disposed in the second wall 38; the edges ofthe openings comprise a slightly incurved shoulder or return 47, 47'.

After having passed a first baffle device 34, the fumes must pass asecond 35 and a third 36, successively.

And these baffle devices 34, 35, 36 are permanently sprayed from thepipes 22, with the result that the streams of the fumes passing throughthe baffle devices pass in fact through a curtain of rain andpermanently sweep the walls 37, 38 which comprise a thin layer film ofthe liquid phase poured from the pipes 22 and constituted by the basicneutralizing slurry.

In this way, a system is obtained in which the interface between the twogaseous and liquid phases represents an extremely large surface,faciliting exchanges and reactions between the two phases, the acidvapours contained in the gaseous phase being called upon to combine withthe basic principles contained in the neutralizing slurry.

And under these conditions, the vapours and fumes which arrive in theright hand part of the neutralizing chamber, when they escape throughconduit 41, are virtually bereft of any acidity.

The neutralizing slurry is accumulated in the lower part of chamber 20where it constitutes a bath from which it is removed via conduit 42 withstrainer 43 and recycled by pump 24' towards the supply circuit 23itself supplied by pump 24 from the lower vat 21.

Supply of chamber 20 with neutralizing slurry may consequently beregulated from pumps 24, 24' whose operation may itself be modulatedfrom the level probe 44 by the servo-control circuits 45, 46.

However, within the scope of a programmation and centralized control, itmay also be provided that the data recorded by the level probe 44 aresent to a central unit adapted to initiate orders as a function of allthe data received, this concerning both the recycling and supply of theneutralizing chamber and the other elements composing the assembly line.

The purified gases emerging from the neutralizing chamber 20 andstrongly laden with water vapour may then be subjected to an operationof condensation in the condensation chamber or condenser 25 where theyencounter in counter-flow trickling water coming from the upper spray(s)34.

The water coming from the spray(s) 34 flows over a known packing such asa packing of the Raschig ring type 35 which may advantageously be of thetype described in French Pat. Nos. 79 30419 and 81 13369.

The trickling water, enriched with the condensation water coming fromthe purified gases, is evacuated at the base of column 25 towards thevat 21 whilst the major part is conveyed from circulator 37 towards alow-temperature heat exchanger 39.

The purified fumes emerge from the top of the tower 25 having lostsubstantially all their calories and may be forced by circulator 49towards the stack 48 for rejection into the atmosphere.

We claim:
 1. A process for exploiting residues or waste of industrial,agricultural or urban origin, for the purpose of recovering thecalorific power of these residues, process comprising the followingsteps of:(a) proceeding by the incineration of said residues or waste ina continuously supplied combustion furnace; (b) recovering thecombustion ashes which are directed towards a vat also supplied withwater so as to form a slurry of basic pH by mixture of the water and theashes; (c) evacuating the high temperature combustion gases anddirecting them towards a heat recuperator, the combustion gases beingmaintained above the dew point until they leave said recuperator, thusavoiding the acidic vapors contained in said bases being harmful to saidheat recuperator, (d) introducing the combustion gases leaving therecuperator, still at a temperature higher than said dew point, into aneutralizing chamber where the gases encounter a flow of theneutralizing liquid constituted by the basic slurry coming from said vatand obtained by mixing a basis of water and combustion ashes thus placedin suspension or solution in said aqueous phase; and (e) evacuating thecombustion gases into the atmosphere, the acid vapors or anhydrides withwhich these vapors were laden having been neutralized, the liquid andsolid residues constituted by the slurry leaving the neutralizing vatalso being neutralized and capable of being recovered to form afertilizer containing mineral salts.
 2. The process of claim 1 whereinthe fumes leaving the phase of neutraliziation are subjected tocondensation to eliminate the water vapor, the water coming from thiscondensation being recovered to constitute with the ashes theneutralizing slurry.
 3. The process of claim 2 wherein the phase ofcondensation of the purified fumes is provided with recycling of the hotwater coming from the condensation of the vapors below the dew point andthis water is recycled to encounter the purified vapors in counter-flow,the hot water before recycling supplying at least one low-temperatureheat exchager (39) to which it transfers its calories.
 4. Aninstallation for carrying out the process of claim 1 which comprises:(a)an incineration furnace (16); (b) a heat recuperator (19) such as atubular boiler in which the combustion gases coming from the furnaceallow production of thermal energy conveyed by a heat-transfer fluidsuch as dry steam or superheated water; (c) a neutralizing chamberreceiving the combustion gases coming from the heat recuperator; (d) atleast one spray pipe (22) located in said chamber; (e) a vat (21) formixing and forming the neutralizing slurry supplied on the one hand withwater and on the other hand with combustion ashes from the incinerationfurnace (16), said vat (21) being connected by a recycling circuit (23)to the spray pipe (22) located in the neutralizing chamber; (f)conveying means connecting the incineration furance 16 to said vat forconveying the ashes; and (g) means (27) for evacuating the solidresidues decanted at the base of the vat.
 5. The installation of claim 4wherein the installation comprises a condenser supplied by the purifiedfumes leaving the neutralizing chamber and laden with humidity, thesefumes arriving at the condenser constituted by a packed column (25)supplied at its top with low-temperature trickling water taking thefumes to a temperature equal to or less than the dew point causingcondensation of the water vapor contained in the fumes, the tricklingwater reheated by heat exchange and increased by the condensation water,at a temperature slightly less than the dew point, being collected atthe base of said packed column and being recycled at the top of thecolumn (25) after passage through at least a second heat exchanger (39)to which it transfers its calories, part of said water collected at thebase of the column being directed towards the vt (21).
 6. Theinstallation of claim 4 wherein the installation further comprises amixer (12) adapted to receive the finest particles having passed thesieving means (10) located upstream of the incineration furnace andwhich are conveyed towards the mixer by conveying means from said sieve,the mixer also being connected by conveying means (27, 32) to the baseof the vat (21) and to the base of chamber (20) and thus adapted toconvey the solid exctracts sedimented in said vat (21) and chamber (20)up to the mixer in order to obtain an organo-mineral improvement.
 7. Theinstallation of claim 4 wherein the neutralizing chamber (20) comprisesa first pipe (22') pouring a neutralizing slurry obtained by suspendingashes in an aqueous phase, this pipe being located at the opening of theconduit (33) through which the fumes penetrate into said chamber.
 8. Theinstallation of claim 4 wherein the neutralizating chamber (20)comprises a baffle device (34, 35, 36) formed by walls (37, 38) of whichthe openings (40, 40') allowing passage of the fumes are offset from onewall to the other, obliging the fumes to follow a complex eddying path.9. The installation of claim 8 wherein the baffle devices are formed bymetal plates (37, 38) made of stainless steel and of which the openings(39, 39', 40, 40') non-concordant from one wall to the other comprise ontheir edges an incurved return (47, 47').
 10. The installation of claim8 wherein the walls forming baffle are sprayed by second pipes (22)pouring over said walls and in the interstitial space between the walls,a plurality of curtains of rain formed by said basic neutralizingslurry.